Scoundrel, endlessly lustful, has one crying plaintively (6)
I believe the answer is:
curlew
'one crying plaintively' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'scoundrel endlessly lustful' is the wordplay.
'scoundrel' becomes 'cur' (cur can mean a despicable person).
'endlessly' means to remove the last letter.
'lustful' becomes 'lewd' (synonyms).
'lewd' with its final letter taken away is 'lew'.
'cur'+'lew'='CURLEW'
'has' is the link.
Can you help me to learn more?
(Other definitions for curlew that I've seen before include "Bird with long down-curved bill" , "Wading bird with fluting call" , "Large wader of the sandpiper family" , "Shorebird of the sandpiper family" , "Curve-beaked bird" .)